Electric motors typically have a stationary portion (“stator”) that energizes a rotational portion (“rotor”) by means of a prescribed signal. For example, DC electric motors typically generate a signal (“commutation signal”) that causes the stator to produce an alternating magnetic field. This magnetic field interacts with permanent magnets in the rotor to cause the rotor to rotate in a controlled manner.
To that end, the commutation signal typically is a periodic signal having a single “on” portion (i.e., the signal is non-zero) and a single “off” portion (i.e., the signal is zero) for every period of the signal. To vary the speed of the rotor, many motors pulse width modulate the on portion of the commutation signal. For example, to reduce the speed by twenty percent from a maximum speed, a prior art motor may pulse width modulate the on portion of the commutation signal so that the average of the on portion is about twenty percent less than that of the on portion of the original commutation signal.
Commutation signals typically are pulse width modulated at frequencies that are much greater than that of the basic commutation signal. For example, the commutation signal may have a frequency of about 200 hertz, while the modulation frequency may be about 20 kilohertz. Undesirably, this can create electromagnetic interference (EMI), which is known to 1) interfere with other electronic devices, and 2) increase the temperature of power devices. To mitigate this interference, many prior art motors using such modulation techniques often have electronic filtering devices. Such additional devices, however, generally increase the cost and power consumption of the motors.